Many boats and other watercraft are propelled by one or more inboard or outboard engines or a stern drive system, which drives one or more propellers. Each propeller typically has three or four blades, but may have as few as two or as many as five or six. The base of each blade is mounted at an angle, or pitch, relative to a radial plane transverse to the axis of rotation of the propeller shaft. Propellers may be constructed with blades having a fixed pitch. The fixed pitch is typically at an angle that provides maximum efficiency at normal cruising speeds, however fixed pitch propellers typically have reduced efficiency at lower vehicle speeds. Alternatively, the pitch of a propeller can be fixed at an angle that provides better acceleration or pulling capacity at lower speeds, which typically results in a reduced top speed. As a result, fixed pitch propellers typically are a compromise between good acceleration, a higher top speed and fuel consumption.
One way to improve the efficiency of propellers at most speeds is to provide a propeller with blades having a variable pitch. One example of a variable pitch propeller is described in U.S. Pat. No. 6,896,564, which is incorporated herein by reference in its entirety. Variable pitch propellers allow for increased efficiency at both low and high speeds, but they are subject to a number of design concerns. Some variable pitch mechanisms are either manually actuated or self-regulated based on either propeller RPM or torque on the propeller blades that urges the blades to a different pitch position against a biasing force. These mechanisms are prone to being inaccurate and do not allow fine adjustment control over the blade pitch. In addition, these mechanisms generally do not allow sufficient pitch variation for the propeller to propel the watercraft in a reverse direction. In addition, variable pitch propeller assemblies must operate within a limited range of size and weight, particularly if it is also desired to vent engine exhaust through the propeller assembly, imposing limits on the size of the actuator and its freedom of movement, thereby imposing a trade-off between finer pitch control and a wider range of available pitch angles.
One example of a variable pitch propeller that addresses some of these deficiencies is described in U.S. patent application Ser. No. 11/962,372, which is incorporated herein by reference in its entirety. While this variable pitch propeller design is adequate, further improvements are possible to provide a simplified, durable mechanism for varying the pitch of the blades, as well as varying the pitch at a non-constant rate.
In addition, outboard engines may be equipped with a transmission disposed in the gear case, to enable the propeller to selectively reverse the direction of rotation of the propeller to propel the boat in the reverse direction, and to provide a neutral position. This transmission adds size and weight to the gear case, resulting in increased drag and fuel consumption. There is a need for a variable pitch propeller capable of propelling a boat in either the forward or reverse direction and providing a neutral position, and having reduced size and weight.
Therefore, there is a need for a variable pitch propeller having reduced size, weight and complexity.
There is also a need for a variable pitch propeller providing both a wide range of available pitch angles and fine pitch control.